What is the absolutely correct blue engine paint color for a 74 Duster?

I cannot answer your question which color is correct, but I can tell you this. I would endeavor to find the correct coed and get it mixed and spray it with a professional sprayer. It sounds like you're going to do that. I've seen so many variances in color from aerosol cans that it's pathetic. I've even seen THREE colors sprayed from THREE different cans with the same part number. It's just stupid how bad quality control is on everything now, so be careful.